How to overdrop a video with black backgroud in the video editor?
To perform a Luma key in the VSE you need to use the Strip modifiers.

However you want to use the source or foreground strip as a key or Mask, and Blender doesn't like making loops (as you are modifying the source with the source which becomes modified etc.).

So you need to duplicate the strip then turn it of with the H key (hide strip). This becomes the Key source strip. You could then Group this pair so that they stay in sync if you move them or trim them on the timeline. In my example I have not done that just so you can see the steps required.



You can better define the value to key by adding a curve modifier to the Key source strip, and make it more contrasty (steeper curve).



For best results you will have to reduce the color of the original strip to make an accurate key. Use the Key source's Saturation slider, reduced to 0.0



For more control you could add a blur effect to the Key source strip then Hide it Alt-H and define the Blur strip as the mask on the foreground strip modifier.



You can also use a curve modifier on the Key source strip to invert the key.

Again I strongly recommend grouping all of these strips together and muting the ones that are used as keys, this saves them accidentally appearing in the composite.